Responses to Gravity and Touch

Plants sense gravity through specialized cells called statocytes located in the root cap. Within statocytes, dense starch-filled organelles called amyloplasts settle downward in response to gravity. This settling triggers a signaling cascade that distributes the plant hormone auxin asymmetrically across the root tip, enabling the plant to detect gravitational direction and grow accordingly.

Positive gravitropism is growth in the direction of gravity, such as roots growing downward into soil. Negative gravitropism is growth opposite to gravity, like shoots growing upward. Both responses result from asymmetrical auxin distribution: in roots, higher auxin on the lower side inhibits cell expansion, causing downward bending; in shoots, higher auxin promotes expansion on the lower side, causing upward bending.

Amyloplasts are starch-filled organelles in statocytes that settle toward the bottom of cells in response to gravity. When roots are vertical, amyloplasts collect at the cell base. If roots rotate horizontally, amyloplasts slide to the new bottom, signaling the plant to reorient. This statolith sedimentation triggers auxin redistribution, enabling gravity sensing and directional root growth.

Thigmotropism is directional growth in response to touch. Roots exhibit negative thigmotropism, growing away from obstacles to follow the path of least resistance. Climbing plant tendrils show positive thigmotropism, growing toward and coiling around objects. This differential response allows roots to navigate soil while enabling tendrils to secure plants to structures.

Tendril tips contain touch-sensitive sensory cells that detect contact with objects. When stimulated, cells on the contact side contract while cells on the opposite side elongate. This differential growth causes the tendril to curve and eventually coil around the object, securing the climbing plant. The rapid, coordinated cell response enables tendrils to wrap tightly around supporting structures.

In vertically oriented roots, thigmotropism overrides gravitropism, causing roots to grow away from obstacles they contact. However, horizontally oriented roots grow downward even when obstacles are present, suggesting gravitropism reasserts control at different angles. This interaction between responses allows roots to navigate complex soil environments while maintaining downward growth.

Statolith sedimentation triggers unequal auxin distribution across the root tip, with higher auxin concentration on the lower side. In roots, elevated auxin inhibits cell expansion on the lower side while allowing rapid elongation on the upper side. This differential growth rate causes the root to curve downward, directing it into the soil in response to gravity.